Some aspects of reaction mechanism study in collisions induced by Radioactive Beams

1. Some aspects of reaction mechanism study in collisions induced by Radioactive Beams Alessia Di Pietro

2. Outline of the talk • Effects on fusion and elastic scattering cross-sections at energies around the Coulomb barrier in reactions induced by halo and weakly bound Radioactive Beams. • What have we learned from the existing experimental data? • The reaction 6He+64Zn. • The reaction 13N+9Be. • Summary and conclusions. • The EXCYT facility at LNS.

3. Effects of Halo structure on fusion below the Coulomb barrier? Different theoretical models predicts enhancement or hindering of fusion cross-section in reaction induced by Halo nuclei depending on how the break-up is treated in the calculations. Break-up removesflux from the fusion channel. Strong coupling with break-up enhances fusion cross-sections.

4. Fusion excitation function Enhancement of fusion cross-section below the Coulomb barrier is observed. J.J. Kolata Phys.Rev.Lett.81(1998)4580 Total reaction excitation function An even larger enhancement of the total reaction cross-section was measured. E.F. Aguilera Phys.Rev.C 63(2001)061603 6He+209Bi

5. Si-Strip Beam 64Zn targets Nb catcher Si-Strip Experimental set-up The experiment 6He+64Zn • Fusion excitation function on a medium mass system. Activation technique used. Elastic scattering angular distribution. Transfer and break-up cross section measurement. Comparison of the results with 4He+64Zn reaction.

6. 6He+64Zn 4He+64Zn 6He+64Zn 4He+64Zn 4,6He+64Zn Heavy residue Excitation Function A strong enhancement of the fusion cross-section seems to be present! CASCADE predictions compared with experimental results The strong enhancement comes only from one residue, 65Zn. 65Zn can be produced also by 1n and 2n transfer reactions. A. Di Pietro et al. Phys.Rev.C 69(2004)044613

7. Experimental set-up R.Raabe et al. Nature 431(2004)823 6He+238U The strong enhancement of the fission cross-section comes from transfer reactions.

8. Elastic scattering For halo projectiles break-up responsible for a damping in the elastic angular distribution at large angles. New features such as the disappearance of threshold anomaly effect of the Optical Potential observed in reactions induced by weakly bound and halo nuclei. The Optical Potentials basic ingredient for the description of elastic scattering but also important for break-up, transfer and fusion. Calculations should be performed using models which take into account explicitly or by Polarisation Potentials the coupling with transfer and break-up channels. Optical Model analysis at low energies performed using very large imaginary diffuseness parameter in order to reproduce data.

9. Elastic scattering s ~1.2b srea0.65 b srea1.45 b Transfer+break-up 6He+64Zn@Ecm=12.4 MeV A.Di Pietro et al. Phys.Rev.C 69(2004)044613 A.Di Pietro et al. Europhys.Lett. 64(2003)309 4,6He+64Zn Large reaction cross-section is found in 6He+64Zn when compared with 4He+64Zn at the same Ecm aI=0.85fm for 6He+64Zn Most of the reaction cross-section corresponds to transfer and break-up events rather than fusion.

10. aI=1.22 fm aI=3.02 fm Other elastic scattering results Very large imaginary diffuseness must be used to best-fit the data 6He+209Bi 6He+208Pb E.F. Aguilera et al. Phys.Rev.C 63(2001)061603R O.R. Kakuee et al. Nucl.Phys.A 728(2003)339 Energy dependent aI parameter: aI=1.964-0.045xEc.m.

11. 9Be+64Zn 6,7 Li+64Zn P.R.S.Gomes et al. Phys.Lett.B 601(2004)20 Effects on reaction and/or fusion cross-section induced by weakly bound nuclei above the Coulomb barrier? Results on medium mass targets 9Be+64Zn shows no effects on fusion and reaction cross-section. 6,7Li+64Zn shows no effects on fusion but large reaction cross-section

12. 9Be+208Pb 6,7 Li+209Bi M.Dasgupta et al. PRL82(1999)1395 M.Dasgupta et al. Phys.Rev.C 66(2002)041602R Results on heavy targets Suppression of fusion cross-section above the barrier of about 70%. This suppression is attributed to break-up which leads to incomplete fusion.

13. Si-strip detector Monolithic detection module Fusion cross-section for the reaction 13N+9Be (weakly bound projectile on weakly bound target) Experimental set-up Detection system based on Si-strip detectors and Monolithic E-E telescopes

14. N E [channels] b Energy [channels] Monolithic Si detectors DE thickness 1mm Identification threshold  300-500keV/A for Z  6-20 G.Cardella et al. NIMA378(1996)262 A.Musumarra et al. NIMA409(1998)414

15. 13N+9Be 10B+12C fus (mb) 1/Ecm (1/MeV) 13N+9Be =45 MeV 10B+12C =42 MeV CASCADE Experiment /fus Z The results are in agreement with CASCADE predictions. No evident suppression of fusion cross-section is present. E*(22Na)  40 MeV /fus Z

16. Summary and conclusions From the data so far collected a clear picture of structure effects of halo and weakly bound nulcei on reaction mechanisms is still not available. The role of the break-up has still to be understood. More theoretical and experimental efforts are needed. The experiments with radioactive beams are quite difficult due to the low intensity of such beams. Our results show that X-ray off-line detection seems to be a good tool to obtain fusion excitation functions in reactions induced by light Halo nuclei on intermediate mass targets.

17. EXCYT installation at LNS MAGNEX

18. Facility scheme

19. RIBs intensity table

21. A. Szanto de Toledo et al. Nucl.Phys. A 679(2000)175 Effects on reaction and/or fusion cross-section induced by weakly bound nuclei above the Coulomb barrier? Example : According to this systematic study there is a hindrance of fusion cross-section in reaction between light weakly bound nuclei above the Coulomb barrier.

22. Activity curve X-ray spectrum T1/2= 67.6 m 67,68Ga67,68Zn T1/2=3.26 d 65Zn65Cu Fusion excitation function measured with an activation technique Evaporation Residues produced in fusion reaction are radioactive and decay by Electron Capture. Discrimination of E.R. by X-ray energies and half-lives.